The present disclosure generally relates to polycarbonates having a high refractive index and methods and compositions for preparing the same.
Polycarbonates are synthetic thermoplastic resins derived from bisphenols and phosgenes, or their derivatives. They are linear polyesters of carbonic acid and can be formed from dihydroxy compounds and carbonate diesters, or by ester interchange. Polymerization may be in aqueous emulsion or in nonaqueous solution.
Polycarbonates have many properties and/or characteristics that are desired in certain instances. These include clarity or transparency (i.e. 90% light transmission or more), high impact strength, heat resistance, weather and ozone resistance, good ductility, being combustible but self-extinguishing, good electrical resistance, noncorrosive, nontoxic, etc. Furthermore, polycarbonates can be readily used in various article formation processes, such as molding (injection molding, etc.), extrusion, and thermoforming, among others. As a result, polycarbonates are used frequently to form a wide variety of products including: molded products, solution-cast or extruded films, structural parts, tubes and piping, lenses, safety shields, instrument windows, and medical devices. Household articles formed from polycarbonates can be produced in a great variety of colors and can be painted, glued, planed, pressed, and metalized and can be used to form precision parts and electronic products.
Other potential applications of polycarbonates require good optical properties like clarity and high refractive index. In this regard, the refractive index (“RI”) is the ratio of the phase velocity of light in a vacuum to that in a specific medium. It is also known as index of refraction or the refractive constant.
The applications of polycarbonates which retain good optical properties include multi-layer diffuser and reflector films, and ophthalmic glasses, among others. Polycarbonates formed solely from bisphenol-A, which have a refractive index of 1.58, are used in some of these applications. It is also desirable to have a polycarbonate with a refractive index greater than that of a polycarbonate formed solely from bisphenol-A.
In U.S. Pat. No. 5,532,331, the entirety of which is hereby incorporated by reference, 4,4′-thiodiphenol has been copolymerized to form a polycarbonate having high refractive index.
In U.S. Pat. No. 5,344,910, the entirety of which is hereby incorporated by reference, the use of 2-phenyl-3,3-bis-(4-hydroxyphenyl)phthalimide (PPPBP), a bisphenol derived from phenolphthalein, is discussed. Copolymers of PPPBP were found to have poor melt stability during melt processing resulting in foamy polymer melts and moldings, and discoloration of the resin during melt processing. Use of PPPBP in high amounts also causes the polycarbonate to become brittle.
In U.S. Pat. No. 5,401,826, the entirety of which is hereby incorporated by reference, 4,4′-dihydroxybiphenyl was used with bisphenol-A to form a polycarbonate. However, the refractive index for this polycarbonate is not enhanced.
Accordingly, it is desirable to produce a relatively transparent polycarbonate with a high refractive index having good processing and mechanical properties.